Installing an intercooler or for that matter a bigger one will drop boost over what your used to seeing. Even going from a stock 7.3 to a 6.0 will drop a bit. But it moves more volume. I've never seen one drop 5-7 but it will probably drop 2-3 for sure.

Yup, drop in psi is normal. But if it's too much, something's up.
I've seen guys rig up caps for their pipes and pressure test the cooler and tubes to see if it's leaking.
Basically get PVC caps the size of your pipes and clamp them in the boots at the turbo and intake. In one of the caps drill and thread a fitting for an air chuck and gauge. Add pressure (35 - 40 psi?) and watch gauge to see if it drops. It should hold good.
Even if there's a small leak, it'll suck psi from you, but 5 psi loss doesn't really sound like a leak...

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